Method of cracking hydrocarbon oils



Dec. 7, 1937. c SCHUTT 2,101,800

METHOD OF CRACKING HYDROCARBON OILS Filed Oct. 29, 1934 5 Sheets-Sheet l L k' a l 3 8 k L g l T1 R i R K a g it 3', *3 3 g 5 04 E .N n u g Q 8 g s Q Coo/er I N VEN TOR. Hermann 615d? Jaw-[W ATTORNEYS.

Pefracfvry fiJe/ Dec. 7, 1937. H. c. SCHUTT METHOD OF CRACKING HYDROCARBON OILS Filed Oct. 29, 1954 3 Sheets-Sheet 2 K9 tmukmq All n Q m OWN ATTORNEYS.

Dec. 7, 1937. H. c. SCHUTT METHOD OF CRACKING HYDROCARBON OILS Filed Oct. 29, 1954 5 Sheets-Sheet 3 ATTO NEYS.

lll l 1 N V EN TOR. Herman/7 Cfich ufi m uQ KUQS QQQU Patented Dec. 7, 1937 METHOD OF CRACKING HYDROCARBON OILS Hermann C. Schutt, North Tarrytown, N. Y., as-

signor, by mesne assignments, to Gyro Process Company, Detroit, Mich., a corporation of Michigan Application October 29, 1934, Serial No. 750,426

2 Claims.

My invention relates to a process of cracking hydrocarbon oils.

My invention is an improvement over the process of cracking disclosed in application, Serial No. 687,985 of John W. Throckmorton and is more particularly an improvement of the cracking process known to the art as the Gyro process.

The Gyro process operates at high temperatures and the constituents of the hydrocarbon oil subjected to pyrolytic conversion is of primary importance. The form of cracking process of which my invention is an improvement is that employing a two-stage cracking, namely a primary stage of cracking known as a viscositybreaking step in which a composite charge of fresh feed and recycled oil is mildly cracked in the liquid phase to convert the heavier fractions which, because of their higher reaction velocity will easily decompose. The viscosity breaking step increases the percentage of the lower boiling point constituents in the hydrocarbon oil. The second stage of cracking is one in the vapor phase, that is, a cracking step in which nothing but oil vapors are converted at high temperatures.

In both cracking steps, there is present in the products coming from the cracking zones higher boiling fractions which have not been converted and in the products from the vapor phase operation', refractory fuel oil formed by the high temperature conversion. I shall hereinafter refer to the liquid phase cracking step, or the viscosity breaking operation, as the primary cracking steipgand to the vapor phase cracking step as the secondary cracking step. The residue from the primary cracking operation is known to the art as primary fuel oil, while the residue from the secondary cracking operation is known to the art as refractory fuel oil. The residues from the primary and secondary cracking steps,-though they may be of the same A. P. I. gravity, will boiling range. It has a high hydrogen to carbon ratio, which expressed in atom number ratio, varies from 1.5 to 1.9.- An example of a primary residue separated from a viscosity breaking, or

primary cracking step, had the following characteristics:

H/C ratio 1.7 Gravity A. P. I 8.8 Viscosity S. Furol 122 1029. B. S. 8: W .2 Boiling range 4 mm. Hg.

1. B. P 170 F. 5% 285 F. 377 F. 30% 578 F. 50% 632 F.

The residue from a vapor phase cracking operation in which oil vapors are cracked at high temperatures, though it may be of the same A. P. I. gravity as the primary fuel oil, is different from the residue of the primary cracking operation, chiefly in its lower boiling range and lower hydrogen to carbon ratio, the latter varying from .9 to 1.3.

For example, a crude, identical to that from which the primary residue above referred to was described, was cracked under vapor phase cracking conditions. The residue separated from the vapor phase cracking step had the following characteristics:

H/C ratio .95 Gravity A. P. I 8.8 Viscosity S. Furol 122 15. B. S. & W .4 Boiling range 4 mm. Hg.

I. B. P 140 F. 5% 180 F. 10% 214 F. 248 F. 308 F. 352 F. 396 F. 444 F. 494 F. 574 F.

It will be seen that the secondary residue is a material whose boiling range and hydrogen to carbon ratio is low, relative to the primary residue. If this material is permitted to be admixed with the recycle stock which forms part of the charge to the viscosity breaking or primary cracking step, coking or carbon deposition will take place in the primary cracking zone, since it is the low hydrogen to carbon ratio hydrocarbons which are susceptible to coking. In the operation of the Throckmorton process, a two stage cracking operation is used, namely a primary liquid phase or viscosity breaking step and a vapor phase or secondary cracking step. The residue from the primary cracking step is flashed with the products of the secondary cracking operation into a flash tower and fractions distilled therefrom to leave a heavy tarry residue. The vapors from the distillation pass into the main fractionating tower where portions of them condense and are recycled together with the fresh feed. The primary cracking step residue, which will be hereinafter referred to as the primary residue, contains valuable cracking stock of favorable high hydrogen to carbon ratio hydrocarbons and high boiling range, which should be recycled in order to maintain the yield and produce a motor fuelof high octane value. The products of the secondary cracking operation contain material which is less desirable for recycling as cracking stock due to its low hydrogen to carbon ratio. A cracking stock whose hydrogen to carbon ratio is low will not only form carbon depositions readily, as has been previously pointed out, but the lower boiling hydrocarbons produced are of extremely poor quality. Then, too, the yield is very low and such hydrocarbons as are suitable for use as a motor fuel have a very low octane number. By referring to the boiling ranges above pointed out, it will be observed that, if the primary residue and the cracked products are distilled in a common tower for the production of a combined fuel oil residue of certain specification, as is the case in the Throckmorton process, the refiner is on' the horns of a dilemma. If he recovers valuable constituents suitable for cracking, from the primary residue by distillation in the flash tower, he will obtain from the products of the cracking reaction a larger proportion of the low hydrogen to carbon ratio constituents productive of low octane value motor fuel. If he distills both the primary residue and the products of the cracking operation mildly, he will lose valuable constituents present in the primary residue and thus reduce his yield, though maintaining a high 00- tane value in the ultimate product.

It is an object of my invention to provide in a process for the conversion of hydrocarbon oils a method of producing in a; given'cracking process a greater yield of motor fuel of high octane value from a given quantity of charging stock than has heretofore been possible.

It is a further object of. my invention to provide in a process for the conversion of hydrocarbon oil, a method of minimizing coke formation in oils being subjected to pyrolytic treatment.

It is a further object of my invention to provide in a process for the conversion of hydrocarbon oils a method for reducing the percentage of undesirable low hydrogen to carbon ratio petroleum hydrocarbons ordinarily returned to the conversion zone with the recycle stock and the fresh feed.

It is a further object of my invention to provide in a process for the conversion of hydrocarbon oil a method for securing a residue fuel oil of predetermined specification without the attendant formation of low grade, low octane value motor fuel of gasoline boiling range.

It is a further object of my invention to provide in a vapor phase process for the conversion of hydrocarbon oils a method for controlling the constituency of recycle stock which will permit higher primary heating temperatures, effect greater vaporization in the primary evaporator and reduce the formation of coke in the primary heating section.

Another object of my invention is to provide a vapor phase cracking process in which the formation of methane is minimized. The compounds having a low hydrogen to carbon ratio produce fixed gases and particularly methane when they are subjected to cracking conditions. Methane is obnoxious in a polymerization process and, accordingly, its formation is sought to be avoided.

Other and further objects of my invention will appear from the following description.

In the accompanying drawings which form part of the instant specification and are to be read in conjunction therewith and in which like reference numerals are used to indicate like parts in the various views:

Figure 1 is a diagrammatic view showing one form of apparatus capable of carrying out the process of my invention.

Figure 2 is a graph showing distillation curves of the primary and secondary residue taken from the primary evaporator and the flash tower 25 of Figure 1, distilled under four millimeters of mercury.

Figure 3 is a diagrammatic view of another form of apparatus capable of carrying out the process of my invention.

In general, my invention contemplates a process of cracking hydrocarbon oils to form low boiling hydrocarbons suitable for use as a motor fuel in which an oil formed in part by fresh charging stock and in part by recycle stock is charged to a cracking coil and cracked in the liquid phase to perform a viscosity breaking operation. The cracked 'oil is then flashed into vapors and unvaporized oil in an evaporator. The vapors are passed to a cracking zone and cracked in the vapor phase at temperatures around 1000 F. The cracked vapors are immediately reduced in temperature by a quenching oil, withdrawn from the fractionating tower. The quenching oil is a fraction heavier than the desired motor fuel and it immediately reduces the temperature of the reacting products to one below active cracking temperature. The quenched mixture passes to a separator in which the vapors are separated and introduced into a fractionating tower for dephlegmation and separation. The residue withdrawn from the separator has a low hydrogen to carbon ratio and is the refractory fuel oil which is separated from the process. The unvaporized oil which has been separated by flash evaporation of the primary cracked products is removed and passed into a still in which desirable constituents are evaporated, either by self-contained heat or by highly superheated steam or any other suitable distillation means. The vapors from the distillation are condensed and the condensate passed to the fractionating tower, or the vapors may be passed directly to the tower, if desired. The fuel oil or tar removed from the primary residue still may be separately withdrawn or admixed with the refractory fuel oil.

More particularly referring now to the drawings, the oil to be cracked, which may be a reduced crude oil, is pumped through line I which is controlled by valve 2, to fractionating tower 3, equipped with the usual fractionating trays 4,

where it is commingled with higher boiling hydrocarbons which have been previously passed through the process and have not been converted. The oil withdrawn through line from the bottom of the fractionating tower, which contains the fresh feed and fractions separated from the process in fractionating tower 3, is pumped by pump 6 to convection tube bank 5' and then through radiant tube bank I in which the oil is heated by radiant heat from burners The oil leaves the radiant tube bank I through line 9, which is controlled by valve Ill, and is flashed in flash evaporator intovapors and unvaporizcd oil, the unvaporized oil forming the primary residue. The vapors pass from the evaporator II which is equipped with trays |2 for better separation, and pass through line l3. through convection tube bank l3, radiant heat bank M in which the vapors are heated to temperatures around 1000' F. by radiant heat from burners I4. The oil then passes through soaking bank 8 to allow for a time of reaction, then through line |5 to arrester H in which the vapors are immediately quenched to below active vapor phase cracking temperature. The quenching oil is removed from the fractionating tower through line l8 by pump |9. A portion of the quenching oil may be bled through line 2| which is controlled by valve 22 for introduction to the flash evaporator to act as reflux to enable better separation to be made in the evaporator. The main body of quenching oil passes through heat exchanger l8 to the arrester, being controlled by valve 20. The quenched products pass through line 23 to refractory fuel oil separator 25, which is equipped with fractionating trays 25. A portion of the quenching oil may be bled through valved line 24 for introduction to the trays in the separator to provide for reflux and to control the extent of the desired separation. The vapors which contain all the desirable constituents of the cracked products, pass through line 26 into the fractionating tower 3. The secondary residue or cracked residue is withdrawn from separator 25 through line 21 which is controlled by valve 28 and is pumped through cooler 21' to storage. It will be noted that the refractory fuel oil or the secondary residue containing the undesirable low hydrogen to carbon ratio compounds is withdrawn from the system and thus prevented from contaminating the recycle oil. The primary residue is withdrawn from the flash evaporator through line 29 which is controlled by valve 33 and passes to still 3| which is provided with a heating means such as superheated steam pipe 3|. The still 3| may be maintained under a vacuum by means of steam jet 32 in order to permit distillation to proceed by self-contained heat, if desired, in which case no steam is supplied through line 3| If desired, a rebo-iler may be fitted and heat may be added, together with a vacuum to assist in distilling the primary residue. The vapors from the still pass overhead through line 33, through condenser 34. into accumulator 35 from which they are removed through line 36 and pumped by pump 31 through line 40 to the fractionating tower, line 40 being controlled by valve 4|. Part of the condensate is bled through line 38 which is controlled by valve 39, as reflux to enable the better separation of the desired constituents. Obviously, the vapors may be delivered directly to the tower 3 by line 33 if desired. The fuel oil obtained from the distillation of the primary residue is removed from the still 3|, through line 42, and is pumped by pump 43 through cooler 44. The primary fuel oil may be separately withdrawn through valve controlled line 44' or may be commingled with the refractory fuel oil as can be readily seen by reference to the drawings.

Referring now more particularly to Figure 3, I have shown another method for effecting the separate distillation of the primary residue. The primary residue flows through the pipe 29 and reducing valve 30 to the still 3|. This still is maintained under a low pressure of, for example, 5-7 lbs. absolute by the suction produced in a Venturi tube 32' at the end of the vapor removal line 33 leading from the still 3|. This still is provided with baffles 80 minimizing the entrainment of liquid with vapors.

The vapor line 26 leading from the refractory fuel oil separator 25 tothe fractionating tower 3 has interposed therein a Venturi tube 32' to the throat of which is connected the vapor line 33 of the still 3|. The vapors pass through the line 28 and Venturi tube 32 in sufficient quantity and under a sufficient pressure head to create a drop in pressure at the throat adequate to maintain the still 3| at a pressure appreciably below atmospheric. The quantity of. vapors evolved in the separator 25 may be for example twenty times that evolved in still 3|. I have taken advantage of the mass effect of this quantity on the relatively small quantity evolved in the still 2| to provide a relatively simple method of distilling the primary residue through pressure reduction and contained heat. If desired, steam may be admitted to the still 3| through the valved line 3| to aid in the stripping of the lower boiling constituents of the primary residue.

The utilization in the Venturi tube of the mass efiect of the vapors evolved in the evaporator 25 is particularly advantageous in any system of cracking since the products from the cracking zone are delivered under pressures ranging from thirty to one thousand lbs. per sq. in. gauge.

It will be apparent that although I have described my invention as applied to a vapor phase cracking process, it is equally applicable to liquid phase, or mixed phase cracking processes. In such processes, separate preheating of the fresh feed as a viscosity breaking operation followed by a primary separation of a primary fuel oil would be practiced before introduction of the feed to the cracking zone. The primary fuel oil could then be separately distilled to recover valuable cracking stock returnable to the fractionating tower. All of the objectionable refractory tar in the cracked products would be removed as a refractory residue.

The cracked vapors are withdrawn from th fractionating tower through line 45 and pass through condenser 46 to reflux accumulator 41 from which reflux is withdrawn through line 48 and pumped by pump 49 to the fractionating tower 3. The uncondensed vapors pass from the reflux accumulator 41 through line 50, through condenser 5|, to separator 52. The gas and uncondensed vapors are removed from separator 52 to line 54 and are compressed by compressor 54'. The liquid hydrocarbons are pumped from the separator through line 53 into the output from the compressor and the combined stream passed through cooler 60, through line 6|, to reflux accumulator 62, from which gases pass through line 63. The liquid hydrocarbons pass through line 64, through heat exchanger 65, line 66, to the stabilizer tower 61. The heat of reboiling for the stabilizing operation is furnished from a hot fraction withdrawn from the main fractionating tower through line 61' and returned thereto through line 68, after passing through reboiler 69. The vapors from the stabilizer tower pass overhead through line ID, are condensed in condenser H, and pass to stabilizer reflux tank 12. Reflux from the tank 12 is pumped by pump 13 through line H to the stabilizer. The cracked, stabilized motor fuel is withdrawn from the stabilizer tower through line 15, is passed through heat exchanger 65 in heat exchange with the incoming unstabilized motor fuel and passes through line 16 to storage. The uncondensed gases or residue gases are withdrawn from stabilizer reflux tank, through line 11 and are withdrawn from the system through line 18 which is controlled by back pressure control valve 19.

It is believed that the operation of my process will be clear from the foregoing description. Because the secondary or highly cracked residue which is of low hydrogen to carbon ratio is withdrawn from the system, there is no danger in coking in the viscosity breaking stage and the production of objectionable fixed gas such as methane is reduced. The motor fuel produced by my process is of exceptionally high octane number and makes a premium motor fuel. By my process, I am enabled to increase the yield without the attendant disadvantages of lowering the octane number and necessitating frequent shut-downs for cleaning because of coke deposition.

My invention recognizes the difficulty with the systems of the prior art and cures the disadvantages. I

It will be understood that certain features and sub-combinations are of utility and may be employed without reference to other features and sub-combinations. This is contemplated by and is within the scope of my claims. It is further obvious that various changes may be made in details within the scope of my claims without departing from the spirit of my invention. It is, therefore, to be understood that my invention is not to be limited to the specific details shown and described.

Having thus described my invention, what I claim is:

1. In a process of cracking hydrocarbon oil to form desired lower boiling hydrocarbons including the steps of passing a stream of hydrocarbon oil through a conversion zoneand subjecting the stream while passing through said zone to liquid phase cracking conditions of temperature and pressure for a period of time suflicient to form lower boiling hydrocarbons from higher boiling hydrocarbons of the oil, reducing the pressure on the products of the cracking operation to effect a separation of the oil into vapors and a primary liquid residue, subjecting vapors separated from the oil to vapor phase cracking conditions of temperature and pressure for a period of time suihcient to form desired lower boiling hydrocarbons from higher boiling hydrocarbons of the vapors, cooling the products of the vapor phase cracking operation with cooler oil sufficiently to condense the refractory higher boiling hydrocarbons of the cracked vapors, separating the condensed refractory higher boiling hydrocarbons from the uncondensed cracked vapors, passing the uncondensed cracked vapors to a fractionating zone, dephlegmating the vapors in the fractionating zone and recovering from the uncondensed overhead products of the fractionating zone the desired low boiling hydrocarbons; the improvement comprising withdrawing from the fractionating zone an intermediate fractional oil condensate lower boiling than said refractory higher boiling hydrocarbons and employing said condensate as said cooler condensing oil and as reflux oil in the separation of said refractory higher boiling hydrocarbons from the uncondensed cooled cracked vapors, diverting the condensed refractory higher boiling hydrocarbons as a secondary residue from the process, passing the primary liquid residue to a separate distillation zone, vaporizing primary lower boiling hydrocarbons of the residue in said zone, separately withdrawing the vapors and unvaporized residue from the distillation zone, cool- I ing the vapors to condense the same, passing condensate resulting from the cooling of said vapors to said fractionating zone and withdrawing from said fractionating zone a stream of hydrocarbon oil for passage to said liquid phase conversion zone.

2. In a process of cracking hydrocarbon oil to form desired lower boiling hydrocarbons including the steps of passing the oil through a conversion zone and subjecting the oil during its passage therethrough to cracking conditions of temperature and pressure for a period of time suflicient to form lower boiling hydrocarbons from the higher boiling hydrocarbons of the oil, discharging the oil from the conversion zone, reducing the pressure on the discharged oil suf ficiently to vaporize a portion of the lower boiling hydrocarbons of the oil while leaving higher boiling hydrocarbons of the oil as a primary liquid residue, passing the vapors through a second conversion zone and subjecting the vapors during passage therethrough to vapor phase cracking conditions of temperature and pressure for a period of time suificient to form lower boiling hydrocarbons in the vapors, discharging the cracked vapors from the second conversion zone, cooling the discharged vapors with a cooler oil sufliciently to condense therefrom refractory higher boiling hydrocarbons of the discharged vapors, passing the cooled uncondensed vapors to a fractionating zone, dephlegmating the vapors and condensing the overhead vapors from said fractionating zone to recover desired low boiling hydrocarbons, the improvement comprising employing as said cooler oil, an oil lower boiling than the said refractory higher boiling hydrocarbons of the cracked vapors, diverting said condensed refractory higher boiling hydrocarbonsas a secondary residue from the process, passing said primary residue to a separate distillationzone,,maintaining a pressure in said last mentioned zone sufliciently low to vaporize lower boiling hydrocarbons of said residue, withdrawing vapors from said last mentioned zone by entrainment with the cooled uncondensed vapors passing to said fractionating zone, the rate of withdrawal of the vapors from said distillation zone being suflicient to maintain the desired low pressure in said distillation zone, and withdrawing reflux condensate from said fractionating zone as the oil to be passed through said first conversion zone.

HERMANN C. SCHU'I'I. 

